1. Field of the Invention
The invention relates to telecommunications. More particularly, the invention relates to a method and apparatus for correcting imperfectly equalized bipolar signals.
2. State of the Art
The first commercial digital voice communications system was installed in 1962 in Chicago, Ill. The system was called "T1" and was based on the time division multiplexing (TDM) of twenty-four telephone calls on two twisted wire pairs. The digital bit rate of the T1 system was 1.544 Mbit/sec (.+-.200 bps), which was, in the nineteen sixties, about the highest data rate that could be supported by a twisted wire pair for a distance of approximately one mile. The cables carrying the T1 signals were buried underground and were accessible via manholes, which were, at that time in Chicago, spaced approximately one mile (actually, 6000 ft.) apart. Thus, analog amplifiers with digital repeaters were conveniently located at intervals of approximately one mile.
The T1 system is still widely used today and forms a basic building block for higher capacity communication systems including T3 which transports twenty-eight T1 signals. The designation T1 was originally coined to describe a particular type of carrier equipment. Today T1 is often used to refer to a carrier system, a data rate, and various multiplexing and framing conventions. While it is more accurate to use the designation "DS1" when referring to the multiplexed digital signal formed at an 8 KHz rate and used to carry twenty-four voice channels by the T1 carrier, the designations DS1 and T1 are often used interchangeably. Today, T1/DS1 systems still have a data rate of 1.544 Mbit/sec and support up to twenty-four voice and/or data DS0 channels. Similarly, the designations DS2 and T2 both refer to a system transporting up to four DS1 signals (96 DS0 channels) and the designations DS3 and T3 both refer to a system transporting up to seven DS2 signals (672 DS0 channels). The timing tolerance for modern T1 equipment has been raised to .+-.50 bps. The T1 and T2 standards are utilized in North America and Japan. Similar, but incompatible, standards called E1 and E2 are utilized in Europe. The T3 standard is utilized in North America and a similar, but incompatible, standard called E3 is utilized in Europe. In the 1980s, fiber optic technology called SONET (synchronous optical network) provided a measure of compatability between T3 and E3 by allowing both to be mapped into an STS-1 signal.
The current standard for T1/DS1 systems incorporates many improvements and enhancements over the original T1 system. The basic T1 system is based on a frame of 193 bits, i.e. twenty-four 8-bit channels (the payload) and one framing bit (F). According to today's standards, the 192 bit payload need not be "channelized" into 24 DS0 channels. In addition, superframe and extended superframe formats have been defined. The superframe (SF) format is composed of twelve consecutive T1 frames, i.e. approximately 1.5 milliseconds of a T1 signal. In the SF format, the twelve framing bits F are divided into two groups, six terminal framing bits F.sub.t and six signalling framing bits F.sub.s. The F.sub.t bits are used to identify frame boundaries and the F.sub.s bits are used to identify superframe boundaries. When the frames are DS0 channelized, the F.sub.s. bits are also used to identify signalling frames. The extended superframe (ESF) format is composed of twenty-four consecutive T1 frames, i.e., approximately 3 milliseconds of a T1 signal. In the ESF format, the twenty-four F bits are divided into three groups. Six F bits are used to provide a 2 kbps framing pattern sequence (FPS) which is used to identify the frame and ESF boundaries. When the frames are DS0 channelized, the FPS is to identify signalling frames. Another six of the F bits are used to provide a 2 kbps CRC (cyclic redundancy check error checking) channel utilizing a CRC-6 code. The remaining twelve F bits are used to provide a 4 kbps data link (DL) channel. The DL channel is sometimes referred to as the "FDL channel" or "FDL link" where DL stands for data link and F stands for facility or facilities.
In addition to modern framing conventions, the present T1 specification also includes provisions for different "line codes", sometimes referred to as "transmission codes". It will be appreciated that the T1 signal is a plesiochronous (tightly controlled asynchronous) signal and, unlike a synchronous signal, is still subject to wander, jitter, and slips. Line codes are signalling conventions which are designed to facilitate frame synchronization and error detection. One popular line code is known generally as alternate mark inversion (AMI or bipolar line code). AMI utilizes a ternary signal (positive, negative, and null) to convey binary digits (zero and one). Successive binary ones are represented by signal elements of alternate polarity and of equal magnitude. Binary zeros are represented by signal elements having zero amplitude. Under the AMI line code, a non-zero signal element which follows a non-zero signal element of the same polarity is called a "bipolar violation".
Prior art FIG. 1 illustrates the bipolar signal for the binary digits 1011. The horizontal lines in FIG. 1 illustrate switching thresholds. When the signal shown in FIG. 1 is received by a "data slicer", the voltage levels are analyzed and if the voltage crosses either threshold, a binary 1 is detected. FIG. 1 illustrates an ideal signal where the timing of the pulses is virtually perfect. Prior art FIG. 4 illustrates how the signal of FIG. 1 appears as "logic levels" to the data slicer receiving it. This virtually perfect signal has correct pulses with correct duration (pulse width).
Signals that are transmitted over coaxial cable or stored on a magnetic medium are susceptible to inter-symbol interference (ISI). ISI occurs when the frequencies making up the transmitted waveform undergo different time delays when traveling to the receiver. The individual pulses become "smeared" together. This makes it difficult for the receiver to determine the correct logic levels. The solution to the problem of ISI is to use an "equalizer" at the receiver which reverses the time delays caused by the transmission medium. An ideal equalizer is a filter having a frequency response which is inverse to that of the medium which caused the ISI. In practice, ISI is variable and the equalizer must constantly adapt, via a feedback network, to the changing frequency response of the transmission medium. Such an equalizer is called an "adaptive equalizer".
Prior art FIG. 2 illustrates the signal of FIG. 1 after it has travelled through a length of coaxial cable. The ISI introduced into the signal distorts the signal by slowing the rise and fall times such that some pulses may fail to cross the switching threshold. Without equalization this signal will be received as having the logic levels shown in FIG. 5. The ISI introduced into the signal will cause it to be misinterpreted as representing the binary digits 1010 rather than 1011.
Prior art FIG. 3 illustrates the signal of FIG. 1 after it has travelled through a length of coaxial cable and after it has passed through an adaptive equalizer. This signal will be received as having the logic levels shown in FIG. 6. Since most equalizers have a high pass frequency response and thus act like a differentiator, transitions are exaggerated. These exaggerated transitions can be seen in FIG. 3 following each pulse. Sometimes, the exaggerated transition can result in a false pulse like the second pulse in FIG. 3. This false pulse is likely to be of shorter duration than a genuine pulse.
In both cases of FIG. 2 (under-equalization) and FIG. 3 (over-equalization) pulses of incorrect duration result, i.e. pulses which are too wide or too narrow (FIGS. 5 and 6). In addition, false bits can occur in the case of overequalization. If pulses are too wide, they may be interpreted as an erroneous bipolar violation by the receiver logic. If the pulses are too narrow, indicating overequalization, two error conditions can occur. First, a narrow pulse representing a valid data bit may be interpreted as a zero by the receiver logic. Second, a narrow pulse produced by an overshoot of the falling edge of a valid bit may be interpreted as a false one by the receiver logic.